INSPECTION METHOD FOR WELDED JOINT

An inspection method for a welded joint formed between a pair of base metals 10, 20 with a groove part 12, 22 and an abutment face 14, 24 being formed on a joint surface Wc between the pair of base metals 10, 20, includes steps of: forming a recessed groove 32 opening to a surface of the base metal 10, 20 in advance at one end of the abutment face; irradiating the joint surface Wc from an X-ray generator 34 placed on a groove part formation side (an exterior space O side) toward the joint surface Wc after at least one pass P1 of build-up welding is performed on the groove parts 12, 22; and determining presence or absence of incomplete penetration in the welded joint part W based on an image formed on a photosensitive film 42 by radiation penetrating the joint surface Wc.

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Description
TECHNICAL FIELD

The present invention relates to a method of non-destructively inspecting a welded joint for incomplete penetration using radiation.

BACKGROUND ART

In a rotor of a steam turbine, in association with the trend of using high temperature steam, a high-temperature section subjected to high temperature steam is made of material different from a low-temperature section subjected to low temperature steam so that each section is made of material appropriate for its environment and the sections are connected to each other by welding. For instance, the high-temperature section is made of high heat resisting steel, whereas the low-temperature section is made of low alloy steel. FIG. 8 illustrates a common structure of a steam turbine. In FIG. 8, the steam turbine 1 is configured such that a plurality of split hollow disks 7 are fitted together between split hollow disks 5 having support shafts 3 and cylindrical ends and that abutment portions of adjacent two of the split hollow disks 5, 7 are connected at a welded joint part W. In this manner, in the steam turbine 1 manufactured by welding the split hollow disks 5, 7 together, it is important to inspect a welding state of the welded joint part W. Further, as illustrated in FIG. 1, the steam turbine 1 has an interior space I formed therein.

FIG. 9A and FIG. 9B illustrate a welding method of related art for the welded joint part W. In FIG. 9A, the base metal 100 and the base metal 200 constitute the hollow disks that are made of different materials and on joint surfaces of the base metals 100, 200, groove parts 102, 202 and abutment faces 104, 204 are respectively formed. As illustrated in FIG. 9A, the groove parts 102, 202 are formed on a side that faces an exterior space O and the abutment faces 104, 204 are formed on a side facing the interior space I. A welding torch 300 is positioned between the groove part 102 and the groove part 202 from the exterior space O.

Next, as illustrated in FIG. 9B, the abutment faces 104, 204 are melted together and also build-up welding is performed on the groove parts 102, 202 one pass at a time using the welding torch 300. After performing the build-up welding, a penetration part M formed between the abutment faces 104, 204 needs to be inspected for lack of penetration. However, the penetration part M cannot be visually checked from the exterior space O. Thus, an inspection hole 106 is formed in the base metal 100 (or the base metal 200) to insert a borescope (not shown) into the interior space I through the inspection hole 106 from the exterior space O in the direction of arrow a. If the penetration part M is accessible, the borescope is inserted toward the penetration part M from the interior space I in the axial direction of the interior space I (direction of arrow b). Then, using the borescope, the penetration part M is visually checked for complete penetration. This visual inspection is preferably performed immediately after welding the first pass (a root pass) to make it easier to repair the part in case that lack of penetration is found.

It is described in Patent Literature 1 to perform the above inspection on the welded joint part of a steam turbine rotor. Further, Patent Literature 1 also describes that an X-ray source is inserted into the interior space I through the inspection hole and then a radiographic test of the welded joint part is carried out to inspect the welding state of the welded joint part. Further, it is described in Patent Literature 2 to visually monitor the welding part, in the case of welding a rotor of a steam turbine or the like, by means of a video system integrated in a welding torch. Furthermore, Patent Literature 3 describes that in the case of welding the steam turbine rotor or the like, the joint surface is formed with a groove and an abutment face and the abutment faces have complementary shapes to form a protrusion and a recess for orienting the joint surfaces of two base metal pieces.

CITATION LIST Patent Literature

[PTL 1]

JP 09-108883 A

[PTL 2]

JP 2010.201507 A

[PTL 3]

JP 2011-177790 A

SUMMARY Technical Problem

In the inspection methods of the related art, it is necessary to form the inspection hole or the interior space I so that the borescope can be inserted to the welded joint part W from the exterior space O. However, the interior space I does not always exist. Also, by making the inspection hole in the base metal, the strength of the base metal is deteriorated and thus, structural design taking into account the strength of the base metal around the inspection hole is required. Further, there is a concern that foreign objects such as steam enter the base metal through the inspection hole, which may affect operation of a device such as a steam turbine formed by the base metals.

As described in Patent Literature 1, in the radiographic test using the X-ray source inserted in the inspection hole, the remaining state of the abutment face is checked using a radiographic image projected on a photosensitive film. Based on this, it is checked whether or not penetration of the abutment face is incomplete. However, the abutment face is hard to form an image on the photosensitive film. Further, the image captured on the photosensitive film is of the part disposed on the side farther from an X-ray generator and thus it is difficult to check presence or absence of the abutment face based on the image on the photosensitive film.

In view of the above issues of the related art, it is an object of the present invention to attain an inspection method for a welded joint, which enables inspection of the abutment face disposed where it cannot be visually inspected, without making an inspection hole in the base metal.

Solution to Problem

To achieve the above object, an inspection method for a welded joint according to the present invention is an inspection method for a welded joint formed between a pair of base metals with a groove part and an abutment face being formed on a joint surface between the pair of base metals. The inspection method comprises steps of:

    • forming a recessed groove opening to a surface of the base metal in advance at one end of the abutment face;
    • irradiating the joint surface from a groove part formation side after at least one pass of build-up welding is performed on the groove part; and
    • determining presence or absence of incomplete penetration in the welded joint based on an image formed on a photosensitive film by radiation penetrating the joint surface.

In the present invention, the recessed groove is formed in advance and the step of irradiating the joint surface is performed after welding. Then, presence or absence of the recessed groove is determined based on the image formed on the photosensitive film. The presence or absence of the recessed groove can be clearly confirmed, unlike the abutment face. If the recessed groove is confirmed, it is determined there is incomplete penetration. If the recessed groove is not confirmed, it is determined that the abutment faces are penetrated. As a result, it is no longer necessary to provide the inspection hole and the above-mentioned issues regarding the inspection hole can be solved. Further, the irradiation step may be performed immediately after welding the first pass (a root pass) so that it is easier to repair the part in case that incomplete penetration is found.

In the present invention, the recessed groove preferably has a cross section area whose dimension has a lower limit so that the recessed groove is distinguishable using the image obtained in the determining step and an upper limit so that the recessed groove is fillable with the at least one pass of build-up welding. As a result, presence or absence of the recessed groove after welding can be confirmed by irradiating the joint surface and the recessed groove can be filled by build-up welding. Therefore, the adverse effect on strength, etc. of the base metal can be eliminated.

A shape of the recessed groove in cross section may be arc, preferably rectangular, so that the image obtained by irradiation can be clearly formed on the film. In the case where the recessed groove has a rectangular shape in cross section, the recessed groove has, for example, an opening width of 0.3 mm or more to 0.5 mm or less and a depth of 0.2 mm or more to 1.0 mm or less to satisfy the above conditions.

In the present invention, the recessed groove is formed preferably on the joint surface of only one of the pair of base metals. This makes it easier to form the recessed groove. Further, on the joint surface, a protrusion and a recess are preferably formed adjacent to the recessed groove, the protrusion and the recess being configured to complementarily fit to each other. This makes it easier to position the joint surfaces of the pair of base materials with respect to each other.

In such a case that the pair of base metals has a hollow cylindrical shape and are welded to each other in an axial direction via the joint surfaces, with application of the present invention to this, presence or absence of incomplete penetration in the abutment face formed facing the interior space of a hollow cylindrical shape can be inspected without making the inspection hole. For instance, by applying the present invention to a welded rotor for a turbine which is formed by the hollow cylindrical bodies welded to one another in the axial direction via the joint surfaces, presence or absence of incomplete penetration in the abutment face formed in the joint surface can be inspected without making the inspection hole.

Advantageous Effects

According to the present invention, it is possible to check presence or absence of incomplete penetration in the abutment faces of the welded joint surfaces where they cannot be visually checked directly, without making the inspection hole in the base metal. This saves extra work for making the inspection hole and also prevents decline in strength of the base metal. Further, the interior space of the base metal is maintained tightly sealed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a welded joint part in relation to a first embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a part of FIG. 1.

FIG. 3 is an enlarged cross-sectional view of section A of FIG. 2.

FIG. 4 is a cross-sectional view of a photosensitive film used in the first embodiment.

FIG. 5 is a cross-sectional view of a welded joint part in relation to a second embodiment of the present invention.

FIG. 6A is a cross-sectional view of a pre-welding joint surface which is the joint surface before welding in relation to a third embodiment of the present invention.

FIG. 6B is a cross-sectional view of a post-welding joint surface which is a joint surface after welding in relation to the third embodiment of the present invention.

FIG. 7A is an image of the pre-welding joint surface captured on a photosensitive film in relation to the third embodiment.

FIG. 7B is an image of the post-welding joint surface captured on the photosensitive film in relation to the third embodiment.

FIG. 7C is an image of a pre-welding joint surface in which a recessed groove is not formed (Comparison Example).

FIG. 8 is a cross-sectional view of a steam turbine rotor taken from the front.

FIG. 9A is a cross-sectional view of a pre-welding joint surface according to a conventional welding method.

FIG. 9B is a cross-sectional view of a joint surface on which one pass is welded according to the conventional welding method.

 DETAILED DESCRIPTION

The present invention will now be described in detail using embodiments shown in the accompanying drawings. It is intended, however, that unless particularly specified in these embodiments, dimensions, materials, and shapes of components, their relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention.

First Embodiment

A first embodiment of the present invention is explained in reference to FIG. 1 to FIG. 4. FIG. 1 illustrates the state where split hollow disks 10, 20 forming a steam turbine rotor are welded together at a welded joint part W. The steam turbine rotor is formed by a plurality of split hollow disks including the split hollow disks 10, 20 that are welded together in the axial direction. Inside the welded joint part W, an interior space I is formed. FIG. 2 and FIG. 3 show a joint surface We before welding.

In FIG. 2 and FIG. 3, the joint surface We has groove parts 12, 22 formed on an exterior space O side and abutment faces 14, 24 formed on the interior space I side. The abutment faces 14, 24 contact with each other.

As illustrated in FIG. 3, a circular recessed groove 32 opening to the interior space I is formed at one end of the abutment faces 14, 24 in the circumferential direction of the split hollow disks 10, 20. The end of the abutment faces 14, 24 where the recessed groove 32 is formed is on an opposite side of the abutment faces 14, 24 from the groove parts 12, 22. The recessed groove 32 is formed by a rectangular notch 16 formed in the abutment face 14 of the split hollow disk 10 and another rectangular notch 26 formed in the abutment face 24 of the split hollow disk 20, that are disposed facing each other. The recessed groove 32 has a symmetrical shape with respect to the abutment faces 14, 24. In this embodiment, the recessed groove 32 has an opening width X in the axial direction, X being 0.3 mm to 0.5 mm and a depth Y being 0.2 mm to 1.0 mm.

As illustrated in FIG. 2, the joint surface We is welded by placing a welding torch 30 in a groove formed by the groove parts 12, 22 from the exterior space 0, directing the welding torch 30 toward the abutment faces 14, 24, and then rotating the split hollow disks 10, 20 in such a state that the split hollow disks 10, 20 are positioned with respect to each other. In this state, build-up welding is performed by the welding torch 30 on the groove formed by the groove parts 12, 22. By laying one pass (a root pass), a penetration part is formed in the abutment faces 14, 24 and the recessed groove 32 is filled. Next, ten to twenty passes of build-up welding are performed to fill the groove formed by the groove parts 12, 22.

After laying one pass, an X-ray generator 34 is placed in the exterior space O to check presence or absence of incomplete penetration in the abutment faces 14, 24 as illustrated in FIG. 1. Further, a high-temperature film cassette 36 is attached to outer circumferential surfaces of the split hollow disks 10, 20 which are 180° out of phase with the X-ray generator 34, at a position to extend over the welded joint part W.

FIG. 4 illustrates the configuration of the high-temperature film cassette 36. The high-temperature film cassette 36 includes a Teflon™ plate 38, a heat-insulating plate 40 made of heat insulating material, and a photosensitive film 42 on which radiation having passed through the welded joint part W is exposed. The Teflon™ plate 38, the heat-insulating plate 40 and the photosensitive film 42 are stacked in this order from a side nearer to the welded joint part W so as to form the high-temperature film cassette 36. When irradiating a specimen, the radiation penetrates the specimen and gradually weakens due to interaction with the specimen. In the case of a welded part, the radiation penetrates well in a region with defects such as blowholes compared to a region without defects. As a result, the region with defects such as blowholes is detected as a dark image on the photosensitive film 42.

In this configuration, X-rays are emitted toward the welded joint part W from the X-ray generator 34 and the photosensitive film 42 is exposed to the X-rays that have penetrated the welded joint part W to form an image on the photosensitive film 42. The image formed on the photosensitive film 42 is the welded joint part W on a side farther from the X-ray generator 34. Thus, if there is incomplete penetration in the abutment faces 14, 24, an image of the recessed groove 32 is formed clearly on the photosensitive film 42.

According to this embodiment, when there is incomplete penetration in the abutment faces 14, 24, the recessed groove 32 is shown on the photosensitive film 42 clearly as a dark image. Thus, it is possible to precisely acknowledge presence or absence of incomplete penetration in the abutment faces 14, 24. Therefore, it is no longer necessary to drill an inspection hole in the split hollow disks 10, 20. This saves extra work for making the inspection hole and also avoids decline in the strength of the split hollow disks 10, 20.

Further, the interior space I can be maintained airtight, so that steam can be prevented from entering the interior space I and affecting the operation of the steam turbine or deteriorating the operation efficiency. Further, as the cross section area of the recessed groove 32 is rectangular, a shape of the recessed groove 32 can be clearly formed on the photosensitive film 42. Further, as the cross section area of the recessed groove 32 has the above described dimensions, the image of the recessed groove 32 can be formed distinctly on the photosensitive film 42 and the recessed groove 32 can be surely filled with one pass of build-up welding. Furthermore, the welded joint part W is irradiated after one pass of build-up welding. This makes it easier to repair the part after confirming incomplete penetration.

Second Embodiment

Next, a second embodiment of the present invention is explained in reference to FIG. 5. In this embodiment, similarly to the first embodiment, a circular recessed groove 44 opening to the interior space I is formed at one end of the abutment faces 14, 24 in the circumferential direction of the split hollow disks 10, 20. The end of the abutment faces 14, 24 where the recessed groove 44 is formed is on an opposite side of the abutment faces 14, 24 from the groove parts 12, 22. The recessed groove 44 is formed by arranging the split hollow disk 10 and a rectangular notch 46 formed only in the abutment face 24 of the split hollow disk 20 to face each other. The rest of the configuration is the same as that of the first embodiment. According to this embodiment, the recessed groove 44 can be formed by machining only the abutment face 24 and thus formation of the recessed groove 44 is easy.

Third Embodiment

Next, a third embodiment of the present invention is explained in reference to FIG. 6A, FIG. 6B, FIG. 7A, FIG. 7B, and FIG. 7C. In this embodiment, similarly to the second embodiment, the recessed groove 44 opening to the interior space I is formed by arranging the split hollow disk 10 and the rectangular notch 46 formed only in the abutment face 24 of the split hollow disk 20 to face each other. In addition to the recessed groove 44, a protrusion 48 and a protrusion 50 are formed adjacent to the recessed groove 44 behind the recessed groove 44 (the exterior space O side) in the abutment faces 24, 14 of the split hollow disks 20, 10, respectively. These protrusions 48, 50 have shapes to complementarily fit to each other.

FIG. 6B illustrates the joint surface We after the first pass P1, the second pass P2 and the third pass P3 of build-up welding are performed. By the first pass P1, the penetration part M is formed in the abutment faces to fill the recessed groove 44. FIG. 7A is an image of the pre-welding joint part W captured on the photosensitive film 42. FIG. 7B is an actual image of the welded joint part W captured on the photosensitive film 42 after the first pass of build-up welding is performed. In FIG. 7A, the image S44 of the recessed groove 44 is clearly shown. In contrast, in FIG. 7B, no image is formed on the photosensitive film 42 and thus it can be seen that the penetration part M is formed in the abutment faces and the recessed groove 44 is successfully filled.

FIG. 7C is an image of the joint part W shown as a comparison example. In this comparison example, the welded joint part W is not formed with the recessed groove 44 and the image of the welded joint part W is formed on the photosensitive film 42. In FIG. 7C, the line of the abutment face is vaguely confirmed. However, it is hard to distinguish from the rest. This makes it difficult to determine whether or not the penetration part M is formed normally after the first pass of build-up welding.

According to this embodiment, the same function effects as the second embodiment can be obtained. Further, as the protrusions 48, 50 that complementarily fit to each other are formed in the abutment face of the split hollow disks 10, 20, sealing effect of the interior space I in the welded joint part W can be enhanced. Furthermore, as the protrusions 48, 50 are formed, it is made easier to position the split hollow disks 10, 20 with respect to each other when welding.

INDUSTRIAL APPLICABILITY

According to the present invention, inspection of the welded joint part can be performed on the abutment faces of the welded joint surfaces located where they cannot be visually inspected, without making an inspection hole in a base metal.

REFERENCE SIGNS LIST

  • 1 Steam turbine
  • 3 Support shaft
  • 5, 7, 10, 20 Split hollow disk
  • 12, 22, 102, 202 Groove part
  • 14, 24, 104, 204 Abutment face
  • 16, 26, 46 Notch
  • 30, 300 Welding torch
  • 32, 44 Recessed groove
  • 34 X-ray generator
  • 36 High-temperature film cassette
  • 38 Teflon™ plate
  • 40 Heat-insulating plate
  • 42 Photosensitive film
  • 48, 50 Protrusion
  • 100, 200 Base metal
  • 106 Inspection hole
  • I Interior space
  • M Penetration part
  • O Exterior space
  • P1, P2, P3 Build-up welding
  • W Welded joint part
  • WC Joint surface

Claims

1. An inspection method for a welded joint formed between a pair of base metals with a groove part and an abutment face being formed on a joint surface between the pair of base metals, the method comprising the steps of:

forming a recessed groove opening to a surface of the base metal in advance at one end of the abutment face;
irradiating the joint surface from a groove part formation side after at least one pass of build-up welding is performed on the groove part; and
determining presence or absence of incomplete penetration in the welded joint based on an image formed on a photosensitive film by radiation penetrating the joint surface.

2. The inspection method for the welded joint according to claim 1,

wherein the recessed groove has a cross section area whose dimension has a lower limit so that the recessed groove is distinguishable using the image obtained in the determining step and an upper limit so that the recessed groove is fillable with the at least one pass of build-up welding.

3. The inspection method for the welded joint according to claim 1,

wherein the recessed groove is formed on the joint surface of only one of the pair of base metals.

4. The inspection method for the welded joint according to claim 1,

wherein on the joint surface, a protrusion and a recess are formed adjacent to the recessed groove, the protrusion and the recess being configured to complementarily fit to each other.

5. The inspection method for the welded joint according to claim 1,

wherein the pair of base metals has a hollow cylindrical shape and is welded to each other in an axial direction via the joint surfaces.

6. The inspection method for the welded joint according to claim 5,

wherein the pair of base metals is adjacent two of hollow cylindrical bodies of a welded rotor for a turbine which is formed by the hollow cylindrical bodies welded to one another in the axial direction via the joint surfaces.

7. The inspection method for the welded joint according to claim 2,

wherein the recessed groove is formed on the joint surface of only one of the pair of base metals.

8. The inspection method for the welded joint according to claim 2,

wherein on the joint surface, a protrusion and a recess are formed adjacent to the recessed groove, the protrusion and the recess being configured to complementarily fit to each other.

9. The inspection method for the welded joint according to claim 2,

wherein the pair of base metals has a hollow cylindrical shape and is welded to each other in an axial direction via the joint surfaces.

10. The inspection method for the welded joint according to claim 9,

wherein the pair of base metals is adjacent two of hollow cylindrical bodies of a welded rotor for a turbine which is formed by the hollow cylindrical bodies welded to one another in the axial direction via the joint surfaces.
Patent History
Publication number: 20140153693
Type: Application
Filed: Mar 15, 2013
Publication Date: Jun 5, 2014
Patent Grant number: 9194820
Applicant: MITSUBISHI HEAVY INDUSTRIES, LTD. (Tokyo)
Inventors: Shin NISHIMOTO (Tokyo), Makoto TANAKA (Tokyo), Kenji KAWASAKI (Tokyo), Hiroyuki ENDO (Tokyo), Kentaro UCHIYAMA (Tokyo), Yoshinobu KOORI (Yokohama)
Application Number: 13/837,487
Classifications
Current U.S. Class: Flaw Analysis (378/58)
International Classification: G01N 23/04 (20060101);